1. Field of the Invention
This invention relates to underfloor heaters which are arranged to form or close an opening in the base of a liquid heating vessel.
2. Background Information
Underfloor heaters for liquid heating vessels are well known and may be divided generally into two categories: sheathed heaters, which have a sheathed resistance element mounted to or formed on the underside of a metal base plate; and thick film heaters which comprise a printed resistance track on the underside of the base plate. In both bases it is conventional to provide an aluminum diffuser plate between the heating element/track and the base plate itself which is usually of stainless steel. Such a diffuser plate serves several functions, an important one of which is to conduct heat from the element to a thermally responsive sensor in order that overheating caused, for example, by operating the heater without any liquid in the vessel, may be sense quickly and action taken to switch off the heater.
In some applications however, it is desirable to use a thermal sensor to measure the temperature of liquid in the vessel through the heated base. This might, for example, be done in order to heat the liquid to or maintain it at a temperature below boiling. An example of a control arrangement which is designed to do just that is the Applicant's U19 series of controls described in EP-A-1233649. In that document there is disclosed the idea of providing apertures in the diffuser plate between the element and the thermal sensor in order at least partially to decouple the thermal sensor from the direct influence of the heating element, thereby allowing it better to follow the temperature of the liquid in the vessel.
The Applicant has realized that whilst this arrangement is indeed effective it cannot give complete isolation of the sensor from the heater since there must always be part of the diffuser plate which connects the location of the sensor to the heating element.
It is an object of the present invention to improve upon or at least provide an alternative to such arrangements and thus when viewed from a first aspect the invention provides an underfloor heater for a liquid heating vessel comprising a base plate, a heat diffuser portion in good thermal contact with the base plate, a heating element in good thermal contact with the diffuser portion and a sensing region for receiving a thermal sensor wherein said sensing region is not directly connected to the diffuser portion and is at least partly surrounded by a wall.